A Fair Weather Friend  

It has been said that weather is one thing mankind can do nothing about. While this is true, our weather forecasting has gotten so sophisticated that even the most severe storms rarely take us by surprise.

Today, luxury liners cross the Atlantic in a few days and the Concorde can cross the same ocean four times in a single day. Any large storm that might threaten our crossing by sea or air is having its every move noted and transmitted by weather FAX to all interested parties. There was a time, however, when a trans-Atlantic crossing took months and anyone embarking on such a journey knew he was departing into the unknown and that there was a good chance that he would never see land again.

Life at sea was tough; storms arose unexpectedly that could crush a hull like so many match sticks. Thus, from the earliest of times, mariners have sought to devise ever better methods of predicting changes in the weather.

Yet, for all the electronic sophistication at our fingertips, two facts remain as true as they were 300 years ago.

Fact number 1: Big storms can do bad things to little boats.
Fact number 2: Concerned boaters need to know all they can about weather so as to avoid fact number 1.

Of all the navigational instruments found onboard a small boat, one of the most important is undoubtedly the barometer. Why so? Because the barometer is the instrument used to show changes in atmospheric pressure and it is by observing these changes that we can know what kind of weather system is heading our way. When the atmospheric pressure is fluctuating very little, it means the weather is apt to remain stable. A pronounced rise in pressure indicates fair weather is soon upon you.

Conversely, a slow but steady drop in pressure signals the oncoming of a storm - a time to turn into the wind and put the pedal to the metal.

But what is this "turn into the wind stuff?" Well, storms are at the center of extreme low pressure systems and since wind rushes toward the center of the storm to replace the air that through convection is rising into the upper atmosphere, it doesn't take anything more than a pennant or weather vane to point yourself toward fairer weather. But how do barometers work? For that, we need to look into a little history. Don't you just hate it when that happens?

 

The principles leading to the development of the barometer were first studied in 1643 by a student of Galileo named Evangelista Torricelli. Prior to Torricelli's work, it was believed that devices such as water pumps worked because they created a vacuum - as in "nature abhors a vacuum" - and that the water simply replaced the air that had been evacuated by suction. This theory fell apart when it was noticed that it was incredibly hard to pump water (with equipment of the day) higher than about 30 feet.

 

Torricelli soon made three important discoveries. First, that even the air we breathe has weight and it was the downward pressure created by this weight that was actually forcing water up into the pumping system. Secondly, that this pressure can be measured. And finally, that this information could be used to make short term weather forecasts!

Any liquid could be used for measuring atmospheric pressure, but some are so light that instruments made to use them would be inordinately cumbersome. Using water, for instance, would require a tube more than 30 feet high! Mercury, on the other hand, was heavy enough to require a tube only about a meter in length. Thus, the Fortin and Kew types of mercurial barometers were born.